The technology for the production of aluminium by the electrolysis of alumina, dissolved in molten cryolite containing salts, at temperatures around 950.degree. C. is more than one hundred years old.
This process, conceived almost simultaneously by Hall and Heroult, has not evolved as much as other electrochemical processes, despite the tremendous growth in the total production of aluminium that in fifty years has increased almost one hundred fold. The process and the cell design have not undergone any great change or improvement and carbonaceous materials are still used as electrodes and cell linings.
The electrolytic cell trough is typically made of a steel shell provided with an insulating lining of refractory material covered by prebaked anthracite-graphite or all graphite carbon blocks at the cell floor bottom which acts as cathode. The side walls are also covered with prebaked anthracite-graphite carbon plates.
To increase the efficiency of aluminium production numerous drained-cathode cell designs have been developed, in particular including sloping drained cathode surface, as for instance disclosed in United States Patents 3,400,061 (Lewis/Altos/Hildebrandt), 4,602,990 (Boxall/ Gamson/Green/Stephen), 5,368,702 (de Nora), 5,683,559 (de Nora), European Patent Application No. 0 393 816 (Stedman), and PCT application WO99/02764 (de Nora/ Duruz). These cell designs permit reduction of the inter-electrode gap and consequently reduction of the voltage drop between the anodes and cathodes. However, drained cathode cells have not as yet found significant acceptance in industrial aluminium production.
It has been proposed to decrease energy losses during aluminium production by increasing the thermal insulation of the sidewalls of aluminium production cells. However, suppression of the thermal gradient through the sidewalls prevents bath from freezing on the sidewalls and consequently leads to exposure of the sidewalls to highly aggressive molten electrolyte and molten aluminium.
Several proposals have been made in order to increase the sidewall resistance for ledgeless cell operation. U.S. Pat. No. 2,915,442 (Lewis) discloses inter-alia use of silicon carbide or silicon nitride as sidewall material. U.S. Pat. No. 3,256,173 (Schmitt/Wittner) describes a sidewall lining made of a honeycomb matrix of coke and pitch in which particulate silicon carbide is embedded. U.S. Pat. No. 5,876,584 (Cortellini) discloses sidewall lining material of silicon carbide, silicon nitride or boron carbide having a density of at least 95% and no apparent porosity.
Sidewalls of known ledgeless cells are most exposed to erosion at the interface between the molten electrolyte and the molten aluminium which accumulates on the bottom of the cell. Despite formation of an inert film of aluminium oxide around the molten aluminium metal, cryolite operates as a catalyst which dissolves the protective aluminium oxide film at the aluminium/cryolite interface, allowing the molten aluminium metal to wet the sidewalls along the molten aluminium level. As opposed to aluminium oxide, the oxide-free aluminium metal is reactive at the cell operating temperature and combines with constituents of the sidewalls, which leads to rapid erosion of the sidewalls about the molten aluminium level.
While the foregoing references indicate continued efforts to improve the operation of molten cell electrolysis operations, none suggest the invention and there have been no acceptable proposals for avoiding cell sidewall erosion caused by reaction with molten aluminium metal.